Circuit calibration using a time constant

ABSTRACT

According to one general aspect, an apparatus includes a first resistor in a first current path of a resistor-capacitor (RC) circuit, the resistor connected to a power source. A variable capacitor is included in a second current path of the RC circuit and operably connected to the power source and a virtual ground generator. A comparison circuit is configured to make a determination regarding a voltage VR across the resistor to a ground relative to a voltage VC across the capacitor to a virtual ground from the virtual ground generator. A control circuit is configured to make an adjustment of a value of the variable capacitor, based on the determination.

TECHNICAL FIELD

This description relates to circuits and the calibration thereof.

BACKGROUND

An RC circuit may include, for example, a circuit with a power or avoltage source (e.g., battery) connected to a resistor (R) and acapacitor (C). RC circuits are found, for example, in many differentelectronic circuits, e.g., filters and/or phase-locked loops, and may beincluded, for example, on microchips (“chips”) or circuit board-levelcomponents. A time constant of an RC circuit, i.e., the RC timeconstant, generally refers to a time needed for a voltage across theresistor/capacitor to rise (with respect to the capacitor) or fall (withrespect to the resistor) to a defined percentage of a final charging ordischarging value of the capacitor. The RC time constant thus depends atleast on the resistance and capacitance, and, more particularly, isgenerally directly related to a size of each of R and C.

Construction and use of many on-chip RC circuits, such as, for example,RC filters, may benefit from an accurate time constant, in order, forexample, to define associated filter transfer functions independentlyof, e.g., process variations and temperature fluctuations. In otherwords, on-chip RC circuits may be constructed and operated with theexpectation that a time constant of an RC circuit will equal R*C, asexpected in the ideal case. In reality, however, actual values of Rand/or C within a given circuit may not match expected values, and,moreover, may change over a period of time (e.g., again, due totemperature fluctuations experienced by the circuit(s)).

Accordingly, RC circuits and related circuits may be calibrated, so thatthe RC circuit behaves in an expected manner in a known amount of time.For example, a variable resistance and/or variable capacitance may beused, so that periodic adjustments may be made to the RC circuit tocause the actual RC circuit components to function in a predictable wayin an expected amount of time.

In one such technique for calibrating an RC circuit, a current mirrormay first be used to make sure that the same current flows through aresistor and capacitor that are otherwise connected in parallel withinthe RC circuit. Then, the capacitance, which may be variable, may beadjusted until the voltages across the resistor and the capacitor equalone another after a time period (i.e., time constant) of R*C from aninitial state of charge/discharge of the capacitor, as may be shown tobe expected for such a configuration.

Such a current mirror, however, may create a large parasitic capacitanceto ground. One technique for minimizing an effect of such a parasiticcapacitance is to use a large capacitance for the capacitor of the RCcircuit (thereby, relatively speaking, minimizing an effect of theparasitic capacitance). In order to have such a large capacitance,however, it may be necessary to dedicate a relatively large area of achip on which the filter is constructed to the capacitor(s) in the RCcircuit. In such cases, compensation for the parasitic capacitance maycome at a cost of valuable and limited chip area.

SUMMARY

According to one general aspect, an apparatus includes a first resistorin a first current path of a resistor-capacitor (RC) circuit, theresistor connected to a power source. A variable capacitor is includedin a second current path of the RC circuit and operably connected to thepower source and a virtual ground generator. A comparison circuit isconfigured to make a determination regarding a voltage VR across theresistor to a ground relative to a voltage VC across the capacitor to avirtual ground from the virtual ground generator. A control circuit isconfigured to make an adjustment of a value of the variable capacitor,based on the determination.

According to another general aspect, a method includes providing aresistor in a first current path of a resistor-capacitor (RC) circuit. Avariable capacitor is provided in a second current path of the RCcircuit. A virtual ground is provided on one side of the variablecapacitor. A voltage VR across the resistor to the ground is comparedwith a voltage VC across the capacitor to the virtual ground after aperiod of time. A value of the variable capacitor is changed if thevoltage VR and the voltage VC are not the same after the period of time.

According to another general aspect, a circuit includes a resistor in afirst current path of an RC circuit. A capacitor is included in a secondcurrent path of the RC circuit. A current mirror is included in thefirst and/or the second current paths that is configured to maintain asubstantially equivalent current in both the first and the secondcurrent paths. An operational amplifier is included having the resistorand the capacitor connected to inputs thereof, and having the capacitorconnected in a feedback loop of the operational amplifier.

The details of one or more implementations are set forth in theaccompanying drawings and the description below. Other features will beapparent from the description and drawings, and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram of an example circuit that may be used forcalibration using a time constant.

FIG. 2 is a circuit diagram of a second example circuit that may be usedfor calibration using a time constant.

FIG. 3 is a circuit diagram of a third example circuit that may be usedfor calibration using a time constant.

FIG. 4 is a chart showing a change in voltage over time in a circuitthat may be used for calibration using a time constant, such as mayoccur in the circuits of FIGS. 1-3.

FIG. 5 is a circuit diagram illustrating a current flow experienced bythe circuit 300 of FIG. 3.

FIG. 6 is a flowchart illustrating an example process for calibrationusing a time constant.

DETAILED DESCRIPTION

FIG. 1 is a circuit diagram of an example circuit 100 that may be usedfor calibration using a time constant. In the RC circuit of FIG. 1, acapacitor 110 has a capacitance (C), which is variable, and controlledso as to perform its functionality in an accurate, constant, and/orcalibrated time period, which in turn allows for predictable andreliable characteristics for the circuit 100 as a whole (and in relationto other, connected circuits). Moreover, a virtual ground generator 138may be used to establish a virtual ground between a terminal of thecapacitor 110 and a current mirror 116, so that a parasitic capacitance124 associated with the current mirror 116 and/or comparison unit 136may be reduced or eliminated. For example, the virtual ground generator138 may be connected so that the capacitance (C) of the capacitor 110 isbetween a voltage source 132 (or other power source) and the virtualground generator 138. Accordingly, a size or space on an associatedmicrochip that is devoted to the capacitance (C) of the capacitor 110may be reduced. Further, a manufacturing cost of the RC circuit 100 maybe reduced, and the RC circuit 100 thus provides an effective andcost-effective solution for calibration.

The RC circuit 100 includes a first current path 118 and a secondcurrent path 120. The current mirror 116 may be used so that a current(I) in the first current path 118 is substantially the same as a current(I) in the second current path 120. The parasitic capacitance 124 toground may exist in the second current path 120, due to a structureand/or operation of the current mirror 116, comparison unit 136, and/orother associated circuits. For example, the current mirror 116 maygenerally be a circuit designed to copy a current flowing through afirst device by controlling the current in another device of a circuit,thereby keeping both output currents constant. In addition, when currentpaths 118 and 120 need to be very precisely tuned, a correspondinglylarge size of the current mirror 116 may be needed to account for anypotential process variations. The parasitic capacitance 124 to theground generally increases when the size of the current mirror 116 islarger, so that increases in a desired precision of tuning paradoxicallyresult in increased parasitic capacitances, as well.

The currents (I) pass through the resistor 106 and the capacitor 110,resulting in voltages VR and VC at nodes 112 and 114, respectively. Thevoltage source 132 is associated with the RC circuit 100. The voltage(s)across the resistor 106 and the capacitor 110 may be forced to be thesame voltage by a feedback mechanism including the comparison unit 136,the control circuit 126, and/or the virtual ground generator 138. If thecurrent mirror 116 causes current in the first and second current paths118 and 120 to be equal, then it may be shown that a time constant ofthe RC circuit 100 is equivalent to R*C. For example, if ΔVR=I*R andΔVC=I*(ΔT/C), which simply represent the essential current/voltagerelationships of resistors and capacitors respectively, then it may beseen that, if ΔVR=ΔVC, as in the assumption above, then ΔT=RC, where ΔTmay thus be seen to represent the time constant of the RC circuit 100.

In practice, temperature or process variations may result in a differenttime constant. That is, for example, an actual value of R and/or C mayvary from expected or ideal values, due to a manner in which R and/or Care made in the RC circuit 100. Nonetheless, a clock 122 with a veryaccurate timing (e.g., a crystal oscillator) may be used so that the RCcircuit 100 may be calibrated to have a precise time constant ΔTc inorder to provide an accurate and reliable transfer function for anassociated RC filter (not shown in its entirety in FIG. 1) or otherRC-circuit based circuits.

For example, the capacitor 110 may be variable or adjustable, and the RCcircuit 100 may include a control circuit 126 that is configured toadjust the capacitor 110 in order to ensure that it functions, or duringcalibration that it obtains an equivalence of VR and VC, after a giventime period. Of course, it should be understood that the capacitor 110is shown as a single capacitor in the example of FIG. 1, for the sake ofsimplicity, but may represent or include at least one capacitor, i.e.,may include an array of capacitors. Moreover, a total value of the (atleast one) capacitor 110 may be adjusted by connecting or disconnectingone or more of the capacitors.

The control circuit 126 may vary a value of the capacitance (C) of thecapacitor 110, e.g., in response to an output of a comparison circuit136. For example, depending on when and whether VR=VC, as determined bythe comparison circuit 136, the control circuit 126 may adjust a valueof the capacitance (C) of the capacitor 110 accordingly, until a valueof ΔT=R*C, as is needed to achieve a successful calibration. At thispoint, the control circuit 126 may report a notification of calibrationto the control circuit 126 or to other components of the system.

In the example of FIG. 1, however, the just-described process forcalibrating the circuit using a time constant are not affected by theparasitic capacitance 124, which is associated with, for example, thecurrent mirror 116. It may be seen that, as described in more detailbelow, the parasitic capacitance is reduced or eliminated, as comparedto a situation where the virtual ground generator 138 is not utilized.With the parasitic capacitance 124 reduced or eliminated, then, for agiven time constant, a size of the capacitance (C) of the capacitor 110may be significantly reduced (of course, in such a case, a size/value ofthe resistor R may need to be increased during design/build time, inorder to maintain a constant/expected time constant). By reducing a sizeof the capacitance (C) of the capacitor 110, an area on the associatedchip may be conserved, and the RC circuit 100 may be constructed, forexample, on a chip, in a reliable and cost-effective manner.

Thus, to calibrate the RC circuit 100, the time constant 128 may beused. The clock 122, for example, may include a crystal oscillator, andmay be an electronic circuit that uses the mechanical resonance of aphysical crystal of piezoelectric material along with an amplifier andfeedback to create an electrical signal with a very precise frequency.

In operation, then, the same currents (I) in the first and secondcurrent paths 118 and 120 are used to charge the resistor 106 and thecapacitor 110 for a given time period ΔTc. If a voltage reading at theoutput voltage node VC 114 is greater than a voltage reading at theoutput voltage node VR 112 (VC>VR) after the time period ΔTc (meaningthat the actual charging of the capacitor during the time period ΔTc didnot achieve calibration of the circuit), then the value of capacitor 110should be decreased. Similarly, if a voltage reading at the outputvoltage node VC 114 is less than a voltage reading at the output voltagenode VR 112 (VC<VR) after the time period ΔTc, then the value ofcapacitor 110 should be increased. If ΔVR=ΔVC after the time period ΔTc,then R*C is equal to the time constant 128 (i.e., to ΔTc), as describedabove, so the circuit 100 performs as expected during the time period ΔTand no further adjustments to the capacitor 110 are needed, and thecalibration may be completed and a notification and/or a set ofresulting calibration codes thereof may be output to the control circuit126, for example, or other system components.

FIG. 2 is a circuit diagram of a second example circuit that may be usedfor calibration using a time constant. Since FIG. 2 is intended merelyto illustrate an example implementation for obtaining a virtual ground218, a full illustration of an operation of the system 100 is notillustrated with respect to FIG. 2 (e.g., elements corresponding to thecontrol circuit 126, the comparison circuit 136, and the clock 122 arenot illustrated in FIG. 2), but are discussed in more detail below withrespect to FIGS. 3-6.

In FIG. 2, then, an operational amplifier 208 is included, and acapacitor 210 is connected in a feedback loop from an output of theoperational amplifier 208 to an input thereof. Meanwhile, the secondinput of the operational amplifier 208 is connected to a node of theresistor 106, as shown. The large open-loop gain of the operationalamplifier 208, together with the negative feedback loop provided by thecapacitor 210, forces essentially the same voltage potential at twoinputs of the operational amplifier 208, which forces a node to operateas the virtual ground 218. Thus, at least the operational amplifier 208and/or the capacitor 210 may be seen to operate as an example of thevirtual ground generator 138. An additional resistor 228 may beconnected between the resistor 106 and a ground 228, as shown.

Accordingly, the description above of calibrating the RC circuit 200continues to apply, e.g., the capacitor 210 may be varied until theactual properties of the circuit 200 exist after being activated for atime period, as defined by the RC time constant. However, due to thevirtual ground 218, a voltage difference across a parasitic capacitance124A and a parasitic capacitance 124B (which may be associated with theoutput of the current mirror 116 and the input of the operationalamplifier 208 respectively) are effectively eliminated, so that theparasitic capacitances 124A and 124B are reduced or eliminated. Due to alow output impedance of the operational amplifier 208, the parasiticcapacitance 124C (which may be associated with the output of theoperational amplifier 208 and the input of the following stages) mayalso be reduced or eliminated.

For example, the current mirror 116 may be implemented using PMOStransistors, which are appropriately biased so as to cause both currents(I) in the first and second current paths 118 and 120 to besubstantially equivalent to one another in the presence of the voltagesource 132. In this example, the part of the parasitic capacitance 124Amay appear across an electrically-conductive region(s) of the PMOStransistor to ground. Of course, other current mirrors may be used, suchas, for example, cascode current sources. In addition, it also includesparasitic capacitances 124B coming from the input capacitance of theoperational amplifier 208.

Because the parasitic capacitances 124A, 124B, and 124C aresignificantly reduced or eliminated, a capacitance value(s) needed forthe capacitor 210 may be reduced, since there is little or no need toattempt to minimize an effect of the parasitic capacitance by sheer sizeof the capacitor 210. Correspondingly, the silicon area on the chipneeded to provide an effective capacitance value for the capacitor 210is considerably small, thereby conserving valuable space on the chipand/or increasing a cost-effectiveness of producing the chip. Of course,for a reduced value of the capacitor 210, it may be necessary toincrease a value of the resistor R 106 to maintain the same timeconstant ΔTc, as referenced above, since the time constant equals R*C.However, the increased size and value of the resistor R 106 is generallynegligible compared to the savings of space obtained from reducing thevalue of the capacitor 210.

When the circuit 200 is being calibrated, the capacitor 210 is charged(or discharged). Due to temperature fluctuations or process variationsin making the resistor 106 and the capacitor 210 of the RC circuit 200.Their values may vary from the ideal or desired ones. Therefore, asdescribed herein, a value of the capacitor 210 may be adjusted until thevoltage drop across the capacitor 210 converges in value with and maybecome equal to the voltage drop across the resistor 106. This resultsin a fixed time constant ΔT regardless of above mentioned componentvariations. At this point the RC circuit 200 is considered calibrated,so the notification of the completion and/or a set of resultingcalibration codes can be sent to, for example to the control circuit 126shown in FIG. 1, or to other system components e.g., filters (notshown). The components that need to be calibrated are required to havethe same RC structure as the calibration circuits to achieve the bestresults. Thus, transfer functions of an associated RC filter may bedetermined with accuracy.

FIG. 3 is a third circuit diagram of a third example circuit that may beused for calibration using an RC time constant. The RC circuit 300generally illustrates a specific example of the configuration of thecircuit of FIG. 2 in the RC circuit 100 of FIG. 1, in which anoperational amplifier is used to establish a virtual ground and therebyreduce or eliminate parasitic capacitances associated with a currentmirror, operational amplifier, and/or other circuit elements.

Thus, circuit 300 includes the current path 118 and the current path120, which establish voltages VR 316, VP 320, and VC 314, as shown. Asalready described, the current mirror 116 may be used so that a current(I) in the second current path 120 is substantially the same as, orequal to, a current (I) in the first current path 118. The circuit 300also includes a capacitor 326, which is usually a capacitor array, anoperational amplifier 328, and a switch 302, which are connected asshown. Voltage node VP 320 may be established at the positive input ofthe operational amplifier 328, while the negative input may beassociated with a voltage node VN 312.

The switch 302 may be, for example, a transistor. If the switch 302 is atransistor, then the switch 302 may be opened or closed by appropriatebiasing of the transistor, so that the transistor transitions betweenstates of being fully on and fully off. In the fully on state thevoltage across the transistor is almost zero (an effective shortcircuit), while in the fully off state may act as an effective opencircuit.

A comparator 310 compares the voltages VC 314 and VR 316 and outputs avoltage at node VO 318. The control circuit 334 is configured to controlthe switch (e.g., using an appropriate control register), so as to openor close the switch 302. For example, when the control circuit 334causes the switch 302 to be ON or closed (state A), then the shorting ofthe I/O of the operational amplifier 328 (i.e., of the capacitor 326)occurs. When the I/O of the operational amplifier 328 is shorted, outputvoltage node VP 320=output voltage node VN 312=output voltage node VC314. (VP=VN=VC). Conversely, when the control circuit 334 causes theswitch 302 to be OFF or open (state B), the input current (I) in thecurrent path 120 starts charging the capacitor 326. When this happens,output voltage node VC 314=output voltage node VN 312−input current(I)*ΔT/capacitor 326 (that is, VC=VN−(I*ΔT/C)).

As also shown in FIG. 3, the control circuit 334 may be used to controla value of the adjustable capacitor 326. For example, the controlcircuit may connect or disconnected additional capacitors (not shown) inseries or in parallel with the capacitor 326, so as to decrease/increasea total value of capacitance seen between VC 314 and VN 312. Of course,other techniques for varying capacitance may be used, and there may beseparate control circuits for operating the switch 302 and the capacitor326.

An operation of the circuit of FIG. 3 is provided below with respect toFIG. 4. Specifically, FIG. 4 is a chart showing a change in voltage overtime in a circuit that may be used for calibration using an RC timeconstant, such as the RC circuit 300 of FIG. 3, and/or other variationsof the circuits of FIGS. 1 or 2. The x-axis of the chart 400 is time T402, which may be measured with reference to the clock 122. The y-axisof the chart 400 shows voltage, e.g., the voltage at node VC 314.

When a switch, such as switch 302 shown in FIG. 3, is closed, VP=VN=VC.Therefore, VP=VC, so the level of voltage node VC 314 on the y-axis ofthe chart 400 remains constant while in state A (switch closed).

When a state transition 406 occurs, the control circuit 334 opens theswitch (turning it off) and the system enters state B. In state B, thecapacitor 326 starts to get charged. Because of the polarity, thecharging of the capacitor is shown as a negative slope in the areabetween the state transition 406 and a state transition 410. Typically,the capacitor is charged for a time ΔTc 408, which may be made veryprecise and may be defined as the inverse of a crystal oscillatorfrequency, for example. After the time ΔTc 408 the switch may be closedand state A is re-entered.

By definition, and by the equations shown above, the time period ΔTc 408is used to charge the capacitor 326. At the end of the duration of thetime period ΔTc 408, VR should equal VC (as determined by the comparator310) if the circuit is calibrated. i.e., the desired results areproduced in the circuit after the time period ΔTc 408, in the context ofwhatever temperature fluctuations or other irregularities may exist,which may cause the value of R*C to not be as desired. Thus, the controlcircuit 334 may be configured to determine whether the actual propertiesof the RC circuit achieve the desired results after the time period ΔTc408.

In the example of FIG. 4, it is shown that the circuit calibration isnot successful between the state transitions 406 and 410, because theslope of the charging of the capacitor is too steep and it does not meetthe voltage VR after the time period ΔTc 408, and so the cycle repeatsitself with a transition to state A, with the switch closed (on). Hereagain the capacitor 326 is short-circuited until adequately discharged,and, during this time, the control circuit 334 may adjust a value of the(variable) capacitor 326 up or down, as described herein, until a statetransition 414 occurs, after which time another cycle of charging thecapacitor 326 for the time period ΔTc 408 occurs. The cycle taking placeafter the state transition 414 represents a successful calibrationbecause after the time period ΔTc 408, VR=VC, so the adjustment to thevalue of the (variable) capacitor 326 was the correct adjustment,resulting in a set of calibration codes, for example, if it is digitallycontrolled. The calibration codes may be used, for example, by circuitsthat require an accurate RC time constant. The present example shows twocycles in the calibration process. In actuality, there may be a numberof cycles that may occur in other instances.

FIG. 5 is a circuit diagram illustrating a current flow experienced bythe circuit 300 of FIG. 3 during charging of the capacitor 326. Circuit500 includes a current path 118 and a current path 120. As described,the current mirror 116 may be used so that a current (I) in the firstcurrent path 118 is substantially the same as, or equal to, a current(I) in the second current path 120. The resistor 330 is chosen so thatthe voltage drop I*R is equal to the charging voltage I*ΔTc/C.

In FIG. 5 the switch 302 is in an open loop mode and the capacitor 326is charging (state B). FIG. 5 is used to illustrate the open loop modecurrent path 502 between a comparator 310 and the current (I) toward thesource. When the capacitor 326 charges to the point that the voltage atthe comparator 310, VC, is equal or substantially equal to the voltageVR at the comparator 310, then the comparator will output a voltage at avoltage node VO 318, so a RC circuit calibration may be determined, asdescribed above with respect to FIG. 4, and again herein below withrespect to FIG. 6.

FIG. 6 is a flowchart 600 illustrating an example process forcalibration using an RC time constant. A resistor is provided in a firstcurrent path (602). A variable capacitor and virtual ground are providedin a second current path (604). Of course, as shown and described above,the virtual ground may be established with respect to the resistor inthe first current path, as well.

Then, the switch 302 controlled by the control circuit 126 may beactivated for a defined period of time, causing a current to appearacross the capacitor (606). The defined time may be the time constantΔTc. In this way, at the end of the defined period of time, the controlcircuit may determine whether VC=VR or, more particularly, whether VC isless than or greater than VR. If, for example, the comparator 310determines that, in fact, VR is not equal to VC after the defined periodof time (608), then the control circuit may determine whether VR isgreater than VC (610), in which case the control circuit may increase avalue of the variable capacitor (612). Otherwise, if VR is less than VC(610), then the control circuit may decrease a value of the variablecapacitor (614). If, after the defined time, VR=VC, then the controlcircuit 334 may output a notification of calibration completion and/or aset of calibration codes (616), e.g., to the control circuit 126 or toother system components.

It should be understood that with respect to the operations involved inFIG. 6, that a waveform, such as that shown with respect to FIG. 4, maybe produced. In FIG. 6, the operation 606 activates a voltage source fora defined period of time. In this example, the defined period of time(time constant) is fixed, so state B would always cover a fixed distanceon the x-axis, but the slope of the line VC 314 in state B would changeeach time the capacitors are adjusted, since the line VC 314 representsa voltage measurement caused by the adjusted capacitors. For example, ifthe operations of FIG. 6 produced a capacitor array that was too smallto achieve the state VR=VC at the state transition 410, then the slopeof the line VC might change. In such case the capacitor would beadjusted, discharged, and/or re-tested for the same time period. Theprocess would repeat until the slope of the line VC 314 met precisely atthe point whose y component is VR 314, at which time the circuit hasbeen successfully calibrated.

While certain features of the described implementations have beenillustrated as described herein, many modifications, substitutions,changes and equivalents will now occur to those skilled in the art. Itis, therefore, to be understood that the appended claims are intended tocover all such modifications and changes as fall within the true spiritof the embodiments of the invention.

1. An apparatus comprising: a resistor in a first current path of aresistor-capacitor (RC) circuit, the resistor connected to a powersource; a variable capacitor in a second current path of the RC circuitand operably connected to a current mirror and a virtual groundgenerator; a comparison circuit configured to make a determinationregarding a voltage VR across the resistor to a ground, relative to avoltage VC across the capacitor to a virtual ground from the virtualground generator; and a control circuit configured to make an adjustmentof a value of the variable capacitor, based on the determination.
 2. Theapparatus of claim 1 wherein the current mirror is configured to causethe first current path and the second current path to have substantiallythe same current.
 3. The apparatus of claim 1 wherein the comparisoncircuit is configured to make the determination, including whether thevoltage VR is substantially equivalent to the voltage VC, based on atime period.
 4. The apparatus of claim 1 wherein the control circuit isconfigured to make the adjustment in response to the determinationindicating non-equivalence of the voltage VR and the voltage VC, after apassing of a time period.
 5. The apparatus of claim 1 wherein thecontrol circuit is configured not to make the adjustment in response tothe determination, when the determination indicates substantialequivalence of the voltage VR and the voltage VC, after a passing of atime period and/or to output a notification of calibration completionand/or a set of calibration codes based on the indication of substantialequivalence.
 6. The apparatus of claim 1 wherein the comparison circuitincludes a comparator having the voltage VR and the voltage VC asinputs.
 7. The apparatus of claim 6 wherein the control circuit isconfigured to make the adjustment, based on an output of the comparator.8. The apparatus of claim 1 wherein the virtual ground generator isconfigured to establish the virtual ground between the current mirror,an operational amplifier, and the variable capacitor.
 9. The apparatusof claim 1 comprising a switch connected in parallel with the variablecapacitor, wherein the control circuit is configured to operate theswitch so as to initiate current in the second current path for use bythe comparison circuit in making the determination.
 10. The apparatus ofclaim 1 comprising an operational amplifier having the virtual groundestablished at an input thereof, and having the capacitor connected in afeedback loop of the operational amplifier.
 11. The apparatus of claim 1wherein the power source includes a direct current (DC) voltage source.12. A method comprising: providing a resistor in a first current path ofa resistor-capacitor (RC) circuit; providing a variable capacitor in asecond current path of the RC circuit; providing a virtual ground on oneside of the variable capacitor; activating a switch for a period oftime; comparing a voltage VR across the resistor to the ground with avoltage VC across the capacitor to the virtual ground after the periodof time; and changing a value of the variable capacitor if the voltageVR and the voltage VC are not the same after the period of time.
 13. Themethod of claim 12 comprising: determining that the voltage VR and thevoltage VC are substantially the same after the period of time; andproviding a notification of calibration completion and/or a set ofcalibration codes of the RC circuit, based on the determination.
 14. Themethod of claim 12 wherein providing the resistor and providing thevariable capacitor comprise providing a current mirror configured tomaintain an equivalence of current in the first current path and thesecond current path.
 15. The method of claim 12 wherein activating theswitch comprises opening and/or closing a switch connected in parallelacross the variable capacitor.
 16. The method of claim 12 whereinproviding a virtual ground comprises: providing an operational amplifierhaving the variable capacitor connected in a feedback loop between anoutput and an input thereof, and having the resistor connected to theinput thereof, wherein the virtual ground is established at the input ofthe operational amplifier.
 17. A circuit comprising: a resistor in afirst current path of an RC circuit; a capacitor in a second currentpath of the RC circuit; a current mirror in the first and second currentpath that is configured to maintain a substantially equivalent currentin both the first current path and the second current path; and anoperational amplifier having the resistor connected to an input thereof,and having the capacitor connected in a feedback loop of the operationalamplifier.
 18. The circuit of claim 17, comprising: a control circuitconfigured to: receive a clock signal to the RC circuit; cause a currentthrough the capacitor while the clock signal is being received; measurea voltage across the resistor and a voltage across the capacitor afterthe clock signal; and determine whether the voltage across the resistorand a voltage across the capacitor are equivalent.
 19. The circuit ofclaim 18 wherein the control circuit is configured to determine that thevoltage across the resistor and a voltage across the capacitor are notequivalent, and in response to adjust a value of the capacitor.
 20. Thecircuit of claim 18 wherein the control circuit is configured todetermine that the voltage across the resistor and a voltage across thecapacitor are equivalent, and in response to output a notification ofcalibration completion.